1. Field of the Invention
The invention concerns an adsorbent that is particularly suited for treatment of mineral, vegetable and/or animal oils and/or fats, especially for removal of coloring matter.
Adsorbents with exchangeable cations are already known for this purpose and belong to three groups in particular:
2. Description of the Related Art
1. Naturally Active Bleaching Clays
So-called fuller's earths have been used since the mid-19th century for purification of vegetable oils, animal fats and mineral oils. These include bentonites (especially Ca bentonites) of preferably acid origin, but also natural mixtures of palygorskite and smectite also preferably of acid origin. These naturally active clays are generally only dried and ground. Relative to their bleaching activity in vegetable and mineral oils there is therefore considerable dependence on natural properties. These clays are modified if necessary with limited amounts of acid so that the adsorption effect can be optimized within limits (EP-A-0 398 636).
2. Acid-Activated Bentonites
Acid-activated bentonites that proved to be superior in action to natural fuller's earths have been on the market since about 1908 (O. Eckart, Die Bieicherde, page 9 (1929), Verlag Serger & Hepel, Braunschweig). Acid-activated bentonites (bleaching clays) are produced by therefore treating bentonite suspensions with hot mineral acids (for example H.sub.2 SO.sub.4, HCl).
The main mineral montmorillonite of the bentonite raw material is a natural, swellable layered silicate constructed of silicate lamellae stacked one on the other. Each lamella consists of two (SiO.sub.4) tetrahedral layers, between which an (Al(OH).sub.4 O.sub.2) octahedral layered is situated. Montmorillonite is a so-called dioctahedral layer silicate, i.e., only two of the three possible octahedral gaps is occupied by aluminum ions. The Al ions in the octahedral gaps can be isomorphically replaced by divalent cations, like Mg ions. Replacement of the trivalent Al ion with the divalent Mg ion leads to a negative excess charge in the crystal. This negative excess charge is equalized between the silicate lamellae by cations, like Na.sup.+, Mg.sup.2+, Ca.sup.2+, so that electrical neutrality overall exists. The cations situated between the layers are exchangeable by other cations, for which reason bentonites are cation exchangers.
During acid activation acid attack occurs in the octahedral layer of the silicate lamella. The cations situated in the octahedral layer, like Al.sup.3+, Mg.sup.2+ and Fe.sup.3+, are dissolved The exchangeable cations after acid activation essentially consist of Al.sup.3+ ions (20 to 50% of the ion exchange capacity (IEC)), Mg.sup.2+ and Ca.sup.2+ ion (about 30%) and Fe.sup.3+ ions (about 10%). Between 10 and 20% of the exchangeable cations are present as H.sub.3 O.sup.+, which was inserted by acid treatment. According to the type of acid treatment the ratio of exchangeable cations can be varied somewhat, but it is not possible to raise the amount of exchangeable Al.sup.3+ (expressed in % of the total IEC) significantly above 50%.
After acid activation a core that still exhibits the montmorillonite structure remains, which is enclosed by a layer of amorphous silicic acid (formerly the tetrahedral layer of the montmorillonite structure) (cf. Ullmann's Encyclopadie dertechnishen Chemie, Vol. 23, page 322 (1983)).
By acid attack the cation exchange capacity of bentonite is reduced, depending on its origin, from 50 to 120 meq/100 g of bentonite as a function of the intensity of acid attack to about 20 to 80, especially 30 to 50 meq/100 g, for which the remaining montmorillonite core is responsible. Ca.sup.2+, Mg.sup.2+, Al.sup.3+ and Fe.sup.3+ ions are preferably situated between the layers as exchangeable cations. Each employed crude clay exhibits an optimum with respect to bleaching activity in vegetable oil relative to the remaining cation exchange capacity after acid treatment. The specific surface area, depending on the crude clay, initially rises with increasing acid activation from about 40 to 90 m.sup.2/ g to about 180 to 400 m.sup.2 /g, but diminishes again with continuing acid attack, because the formed amorphous silicic acid is crosslinked to chain and ring structures. The specific surface area is also decisive for the action of bleaching clays in vegetable oils and exhibits an optimum that is influenced by the intensity of acid attack.
The pore volume of crude bentonite is also significantly altered by acid attack. Crude bentonites have a pore volume of less than 0.1 ml/g, distributed in pores with a diameter from 0 to 80 nm. Crude bentonites that already exhibit natural bleaching activity have pore volumes between 0.15 and 0.3 ml/g. With increasing acid attack the pore volume is increased primarily in the favor of smaller pores up to a diameter of 14 nm. Increasing bleaching activity in vegetable oils can also be achieved in this range. With continuing acid activation the percentage of larger pores (&gt;25 nm pore diameter) is increased at the expense of smaller pores. At the same time a reduction in bleaching activity is observed. The degree of acid activation therefore also has an effect via the pore volume on the bleaching activity of an adsorbent.
The conclusion is therefore reached for production of acid-activated bleaching clays that good bleaching activity in vegetable oils is only obtained when acid attack is run far enough that optimum ion exchange capacity, specific surface area and pore volume are produced in the product.
The presence of H.sup.+ ions and Al.sup.3+ ions is one of the essential criteria for evaluating a good bleaching clay. These ions are exchanged during treatment with acid between the layers. This occurs in a manner so that Al.sup.3+ is initially dissolved out with the acid from the montmorillonite crystal to then be exchanged partially between the layers via cation exchange reaction. Exchange of H.sub.3 O.sup.+ ions occurs directly during acid treatment.
There is a need to further increase the effectiveness of bleaching clays in vegetable oils in order to permit more limited use amounts in oil.
3. Synthetic Bleaching Clays
EP-A-0 269 173 describes production of synthetic, heat-regenerable bleaching agents and their use for bleaching vegetable oils. A hydrogel is produced from Na silicate and H.sub.2 SO.sub.4 to which an Al.sub.2 (SO.sub.4).sub.3 solution is added during crystallization. The product is dried and calcined. The high ion exchange capacity (IEC) of the product is 43 meq/100 g, in which 32% pertains to Na.sup.+ ions, 65% to Al.sup.3+ ions and 4% to Ca.sup.2+ ions. An increase in the percentage of Al.sup.3+ ions by controlling synthesis is not possible. The bleaching activity of the product is acceptable in linseed oil and soybean oil, but the bleaching agent is not suitable for treatment of palm oil.
Metal silicates are described in DE-A-2 036 819 as synthetic bleaching agents. For example, an Al silicate is produced by heating Al.sub.2 (SO.sub.4).sub.3 with water glass. The products of precipitation, which consist primarily of metal hydroxides, are converted by drying to metal oxides that are then ground. The composition is set within narrow limits by the precipitation process; an ion exchange after synthesis is not described. The bleaching agent exhibits acceptable bleaching activity in linseed oil and soybean oil, but not in palm oil.
The most important clays and clay minerals for production of bleaching earths, as well as the properties of bleaching earths and their use for refining of oils, are described in Ullmann's Encyclopadie der technischen Chemie, Vol. 23 (1977). General comments are made concerning the ion exchange capacity of bleaching clays; no assertions are made concerning composition, let alone the effect of different intermediate layer cations. No mention of incorporation of trivalent ions in particular can be found.
The underlying task of the invention is to improve the bleaching action of natural bleaching clays, acid-activated smectites and synthetic bleaching clays, especially in terms of bleaching of palm oil.
It was surprisingly found that the bleaching activity of these substances, to which ion exchange capacity is common, can be improved if cations that act as Lewis acids, especially Al.sup.3+ ions, are exchanged in the exchangeable sites. Moreover, the following can be exchanged as cations that act as Lewis acids: Zr.sup.3+/4+, Sn.sup.2+/4+, Zn.sup.2+, Ti.sup.3+/4+, Co.sup.2+, Ni.sup.2+, Fe.sup.3+, Cr.sup.3+ B.sup.3+ Mn.sup.2+/3+/4+.